This invention relates to power supply apparatus for supplying electrical power to a number of remotely situated telecommunications devices, such as telephones, and to an assembly comprising such apparatus connected to a plurality of telephones.
A conventional telephone exchange can serve a large number of telephones connected to the exchange along corresponding lines which are of differing lengths as a result of differences in the distances of the telephones from the exchange. As well as relaying telecommunications signals, the exchange also feeds to each line sufficient power to operate the telephones. That power can vary from one telephone to another in view of the differing lengths of the lines, which give rise to differences in the amount of resistive losses on the various lines.
The variation in power requirements is accommodated by means of a number of subscriber line interface circuits (SLICS), each of which relays through a respective line a sufficient amount of power to operate its respective telephone. If linear SLICs are used, excess power supplied to each circuit from the source is dissipated as heat. In this case, the voltage applied to each SLIC from the power source has to be sufficient to operate the telephone connected by the longest line. If a large number of other telephones are connected on much shorter lines, a relatively large amount of power has to be dissipated by the SLICs associated with the shorter lines, which can be uneconomical and leads to cooling problems in the exchange.
Some exchanges use SLICs which have individual internal switch mode power supply units which reduce the power dissipation. However, such SLICs are relatively expensive.
According to the first aspect of the invention, there is provided power supply apparatus for connection to a plurality of remotely situated telecommunications devices along corresponding lines, the apparatus comprising a plurality of interface circuits, each having an input for connection to a common power source and an output for connection to a respective line, each interface circuit, when active, feeding sufficient power, derived from the source, to the line to provide the required amount of power to operate the respective telecommunications device, dissipating any excess power supplied to the interface circuit from the source, wherein the apparatus includes common power regulation means for monitoring a characteristic of the amount of power being fed by each active interface circuit to its respective line and so controlling the power supplied to each active interface circuit that said supplied power is substantially equal to the largest of the amounts of power required by the active interface circuits.
Thus the power regulation means ensures that the power supplied to all the interface circuits is governed by the requirements of the active interface circuit which has to feed the most power to its respective line. The invention therefore helps to reduce the amount of power which needs to be dissipated by the active interface circuits, particularly when the interface circuits with the highest power requirements are not active, and thus enables the apparatus to use relatively cheap linear SLICs, whilst avoiding, or at least ameliorating problems arising from the power dissipation in the SLICs.
Conveniently, the characteristic monitored by the regulating means is the respective voltage applied by each active interface circuit to its respective line, and the regulating means controls the power supplied to the interface circuits by controlling the voltage applied to the inputs of the interface circuits.
Conveniently, the apparatus is adapted to be connected to a source of dc power, for example, a battery.
The power regulation means preferably includes a switch mode power supply unit operable to reduce the voltage supplied by the dc source to the interface circuits.
Preferably, the interface circuits are mounted on a single card for mounting in a rack.
Preferably, the apparatus has a plurality of such cards, and the regulating means is one of a plurality of such means each associated with a respective card.
The use of a plurality of cards and associated regulating means enables -he allocation of lines to interface circuits to be such that the interface circuits on each card are connected to a number of lines of similar lengths, thereby reducing the range of powers to be fed by the interface circuits in each respective card. That reduction in range enables the amount of power dissipated by the interface circuits to be further reduced.
According to the second aspect of the invention, there is provided a telephone exchange having power supply apparatus in accordance with the first aspect of the invention connected to a plurality of remotely situated telephones along corresponding lines which are not necessarily of the same resistance, and to a dc power source.
Preferably, the apparatus has a plurality of cards, each having a respective set of interface circuits and all the lines connected to the circuits on each cards are of lengths which lie in a respective one of a plurality of different ranges.
The invention also lies in power regulation means for use in apparatus in accordance with the first aspect of the invention, the power regulation means comprising a power input for connection to a battery, a plurality of further inputs, each for connection to the output of a respective interface circuit, monitoring means connected to said further inputs for monitoring the voltage, or current fed, by each active interface unit to a respective line connecting that interface unit with a respective telecommunications device, the monitoring means being operable to determine, at any one time, the maximum of said voltages or currents; and control means for applying to all said interface circuits an output voltage, derived from said battery voltage, substantially equal to the largest of the voltages required by an active interface circuit.
Preferably, the monitoring means is arranged to monitor the voltages applied by said active interface circuits and to determine the largest of said voltages.